We can refer to "Unsupported HTML Attributes Compared with the HTML 4.01 Specification" of the “Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 1 of 2)” article, it clearly states that what all is supported/unsupported.

One can use either of the following workarounds:

Steps to view HTML controls in mail item on Outlook 2007

· Open the Outlook Mail Item (Double click on the mail item).

· Click on “Other Action” on Message Ribbon.

· Select “View in Browser”.

· A new browser window will open with mail body having all HTML controls.

Or

Set AutoPreview (on by default) on Outlook2007. If you add your HTML as an attachment to the message, a user could click on the attachment. It would then be “previewed” in the message and the controls are displayed correctly.